Modern vehicles often incorporate both two-wheel and four-wheel drive modes, wherein power may be selectively distributed to two wheels in the two-wheel drive mode and four wheels in the four-wheel drive mode. Vehicles of different sizes often incorporate two-wheel drive and four-wheel drive for the purpose of enabling better handling during varying traction conditions by shifting to the four-wheel drive mode, while still being able to switch to two-wheel drive when less traction is required to reduce fuel consumption and reduce wasted power.
For vehicles with switchable drive modes, devices and systems are needed for engaging and disengaging drivetrain components such as axles and shafts. For example, many vehicles having both two-wheel and four-wheel drive modes, include a transfer case for selectively distributing power output from the engine to front and rear wheels. In particular a transfer case disconnect, such as a clutch, may be included within or proximate to the transfer case for switching between the two-wheel and four-wheel drive modes. In the two-wheel drive mode, the transfer case disconnect may disengage with a secondary driveline to cut torque transmission to the secondary driveline, thereby only providing torque to the primary driveline and its wheels (e.g., either the front or rear wheels). Together, the transfer case and disconnect may transmit power output by the engine towards only two of the wheels, either the front wheels or rear wheels, during two-wheel drive mode, and towards all four of the wheels during four-wheel drive mode.
Some vehicle powertrains may include additional disconnect devices that can be placed in a variety of areas, including at the wheel ends, at one or more axles, or along one of the drive shafts, for more targeted torque delivery to each of the four wheels. For example, a powertrain may include disconnect devices in the front and/or rear axle to selectively transmit torque to each of the wheels on the front or rear axle. In particular, additional disconnect devices may be included in the secondary driveline. During the four-wheel drive mode, the secondary driveline disconnect further apportions torque received from the transfer case amongst components of the secondary driveline. For example, the secondary driveline disconnect may be positioned between the transfer case and one of the wheels of the secondary driveline to selectively transmit torque to either: only one of the wheels of the secondary driveline, or both of the wheels of the secondary driveline. By including one or more disconnect devices in the front and/or rear axle, the relative amount of torque delivered to each of the wheels of an axle may be varied. The disconnect devices often involve some form of a clutch that can move to connect or disconnect two rotatable components such as two shafts. For example, the disconnect devices may include a motor, electromagnetic coil, or other actuator that moves the clutch to connect or disconnect two rotatable components. Through the use of disconnect systems, vehicles can be made more versatile by having the ability to switch between different drive modes depending on the driving conditions and operator desire.
Typically, when a switch to 4×2 mode is commanded, the transfer case clutch moves to a disengaged position, to stop torque transmission to one of the front or rear axle. Further, when switching to, or when in a 4×2 mode, it may be desired to disengage the disconnect devices to reduce frictional losses and fuel consumption. However, the inventors herein have recognized that disconnect devices positioned in the secondary driveline may not disengage when the transfer case clutch is disengaged. Thus, the secondary driveline disconnect devices may remain engaged after the transfer case clutch is disengaged in what is referred to herein as “pinch torque.” As such, components of the driveline that remain coupled by the engaged disconnect devices, such as a propeller shaft, differential, and one or more half shafts may continue to spin when in the 4×2 mode. Thus, fuel consumption may be increased in examples where the disconnect devices fail to disengage when the transfer case shifts to the 4×2 mode, due to the increased frictional losses incurred from the spinning driveline components connected by the engaged disconnect devices. The inventors herein have developed various approaches to address these issues.
Thus in one example, the above issues associated with disconnect systems may be at least partially addressed by a method comprising in response to a desired shift from a four-wheel drive mode to a two wheel-drive mode: decreasing a transfer case torque output to a secondary driveline to a lower first level and disengaging a disconnect device of the secondary driveline; increasing the transfer case torque output from the lower first level to a higher second level over a duration; and after the duration, reducing the transfer case torque.
In this way, by increasing torque input to the driveline disconnect clutch when the clutch remains engaged, the clutch may be released to the disengaged position. Thus, increasing the torque input to the driveline disconnect clutch after initially decreasing the torque input ensures that the disconnect clutch disengages and decoupled two rotating shafts. Specifically, the driveline disconnect clutch collar teeth may slide freely out of engagement with one of the shafts and may not be pinched due to residual torque and prevented from sliding and shifting to a disengaged position. The brief ramp-up of the transfer case torque releases the driveline disconnect clutch collar free of any residual torque, thereby shifting freely and without delay. By disengaging the disconnect clutch during a two-wheel drive mode, frictional drag losses and thus fuel consumption may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.